Heretofore, various silane coupling agents have been publicly known. For example, there exists a fluoroalkyl group-containing silane coupling agent having an NCO group that is reactive with a tooth surface. Those expressed by the general formula (9) shown below are publicly known as the specific examples (see Patent Document 1).CnF2n+1—(X)—SiR3-m(NCO)m   (9)[In the formula (9), X represents (CH2)w or C6H4(CH2)w; w represents an integer of 1-4; R represents an alkyl group; n represents an integer of 1-20; and m represents an integer of 1-3.]
A technology is also publicly known in which a sol based on a silane coupling agent having a fluoroalkyl group is coated on an oxide film based on SiO2 formed by a sol-gel process (see Patent Document 2). The silane coupling agent is a fluoroalkyl silane.
Furthermore, an agent to suppress adhesion of polluting substances has been proposed in which a silane coupling agent having fluoroalkyl groups, long chain alkyl groups, and the like is fixed to a base material on which surface concavity and convexity have been formed (see Patent Document 3).
However, conventional silane coupling agents lack heat resistance, and thus heat resistant silane coupling agents usable at high temperatures have been required.
Accordingly, the present inventor has invented a silane coupling agent having a biphenylalkyl group expressed by the general formula (10) shown below (nF2P2S3M) and filed a patent application (see Patent Document 4). The silane coupling agent has high heat resistance so that its contact angle does not decrease even after exposure to an atmosphere of 300° C. for 2 hours or longer.
[In the formula (10), Rf represents a perfluoroalkyl group of F(CF2)n; and n represents an integer of 1-12.]
It has been found from subsequent research that the silane coupling agent expressed by the general formula (10) is a mixture of an α-adduct and a β-adduct.
The structural formulas of the α-adduct and β-adduct of nF2P2S3M are shown in FIG. 1.
The ratio between the α-adduct and β-adduct changes depending on carbon number of perfluoroalkyl groups and reaction temperature.
It was also difficult to separate the α-adduct and β-adduct since their boiling points are close.
When the mixture is directly used for surface modification of glass, high heat resistance, durability, releasability, and antifouling property are derived due to π-π interaction (π-π stacking) between benzene rings at the modified surface; however, it has been found that the structure of the modified surface is disordered by the α-adduct and the interaction between benzene rings is considerably weakened.
FIG. 2 shows a schematic view representing the surface modified by 8F2P2S3M in which n=8 in the general formula (10) described above.
It has also been found that only the α-adduct is formed when the reaction temperature during synthesis is of no lower than 100° C.
Patent Document 1: Japanese Unexamined Patent Application No. H09-315923
Patent Document 2: Japanese Unexamined Patent Application No. H10-265242
Patent Document 3: Japanese Unexamined Patent Application No. 2000-342602
Patent Document 4: Japanese Unexamined Patent Application No. 2004-107274
Non-Patent Document 1: Journal of Fluorine Chemistry 127, (2006) 1058-1065
Non-Patent Document 2: Dec. 22, 2006, published by Johokiko Co., 1st edit., “Development of Water-Repellent, Hydrophilic, Antifouling Agent and Control of Coating and Wettability”, “3rd section, Development and Performance Advances of Fluorine-Containing Coating Agent”, pages 108-121